Effect of structural and magnetic transitions inLa1−xMxMnO3(M=Sr,Ca) single crystals in Raman scattering

Abstract

Polarized Raman spectra of undoped and doped ${\mathrm{La}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{MnO}}_{3}$ single crystals have been studied in the temperature range from 5 to 423 K ($M=\mathrm{Sr},$ Ca). The spectral intensity is mainly related to scattering by optical phonons at $q\ensuremath{\sim}0$ (for $x\ensuremath{\approx}0$) and the density of vibrational states for $xg0.1.$ Raman spectra from the undoped ${\mathrm{LaMnO}}_{3}$ structure distorted by the static Jahn-Teller effect are consistent with the orthorhombic ${D}_{2h}^{16}$ space group. Raman spectra of the doped ${\mathrm{La}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{MnO}}_{3}$ crystals have weakly resolved features and intensities comparable to those of the second-order spectra in cubic media. The anomalies in the phonon spectra of doped crystals were explained by the doping dependence of the ionic radius of a La/Sr site. The most intense ${B}_{1g}$ mode in ${\mathrm{LaMnO}}_{3}$ crystal has an abrupt frequency shift near ${T}_{N}=140\mathrm{K},$ which corresponds to a paramagnetic to canted-antiferromagnetic phase transition.